The HEVC standard is similar to the H.264 previous standard, in the sense that it can use a block-wise representation of the video sequence.
In a similar manner to the H.264 standard, the aforementioned HEVC standard implements:                an intra prediction which consists in predicting pixels of a current image with respect to already coded and then decoded pixels of this current image,        an inter prediction which consists in predicting the motion or the pixels of a current image with respect to one or more images of the sequence which have already been coded, and then decoded. Such images are conventionally called reference images and are preserved in memory either at the coder or at the decoder.        
More precisely, the images are cut up into blocks of pixels called CTUs (the abbreviation standing for “Coded Treeblocks Unit”) which are similar to the macroblocks of the H.264 standard. These CTU blocks can thereafter be subdivided into smaller blocks called CUs (the abbreviation standing for “Coding Unit”), each of these CU blocks or each CTU block being coded by intra or inter image prediction.
Conventionally, the coding of a current block is carried out with the aid of a prediction of the motion or of the pixels of the current block, delivering a predictor block or else one or more predictor motion vectors, and of a prediction residual, corresponding respectively to a difference between the current block and the predictor block or between the motion of the current block and the predictor motion. This prediction residual is transmitted to the decoder, which reconstructs the block or the current motion vectors by adding this residual to the prediction.
In the HEVC standard for example, when a CTU block is cut up into CU blocks, a data stream, corresponding to each CU block, is transmitted to the decoder. Such a stream comprises:                residual data which are the coefficients of the quantized residual CU blocks and optionally, during a coding in Inter mode, residual data of the motion vectors,        coding parameters which are representative of the mode of coding used, in particular:                    the mode of prediction (intra prediction, inter prediction, prediction skip carrying out a prediction for which no information is transmitted to the decoder (“skip”));            information specifying the type of prediction (orientation, reference image, . . . );            the type of subdivision;            the type of transform, for example 4×4 DCT, 8×8 DCT, etc. . . .            etc. . . .                        
The inter prediction of a current block consists in predicting certain coding information associated with this current block, such as for example:                the temporal motion vector of the current block with respect to a temporal motion vector associated with a block of reference pixels,        the pixels of the current block with respect to the pixels of a block of reference pixels, which is situated in an image other than the current image to which the current block belongs.        
In the case of video of 2D type, this other image is for example an image situated at a previous instant to the current instant at which the current image is situated.
In the case of video of 3D type where the current image is considered in a given view, this other image can belong for example to a view other than the given view. Said other view is for example situated at the same instant as said given view and represents the same scene as the given view, but from another viewpoint.
In the case of video of 3D type where the current image is considered to be a texture component and a prediction of motion of the current image is implemented with respect to a motion vector determined in an image other than the current image, this other image is for example a depth component associated with said texture component, the current block and the reference block having identical positions respectively in the texture component and in the depth component.
Being as it is the prediction of coding information such as a temporal motion vector, there is proposed in the HEVC norm the AMVP (for “Advanced Motion Vector Prediction”) mode which introduces a list of candidate temporal motion vectors to predict the temporal motion vector of the current block. Only the difference (residual) between the motion vector of the current block and the best candidate temporal motion vector of the list in the sense of a bitrate-distortion criterion, and an index signaling the position of this better candidate vector in the list, are transmitted to the decoder, thus reducing the cost of transmission of the information related to the temporal motion vector of the current block.
The prediction in the AMVP mode is not very precise since all the vectors of a current block are predicted with the aid of a single predictor motion vector. Moreover, the fact of transmitting the residuals of motion vectors obtained in the data stream destined for the decoder is expensive in terms of bitrate. It follows from this that the performance of the video coding according to this mode is not optimized.
The HEVC norm also proposes the MERGE mode which avoids predicting an item of information regarding coding of a current block, such as for example a temporal motion vector or a disparity motion vector. It is recalled here that a disparity motion vector is a motion vector used within the framework of an inter-view prediction, in video of 3D type. It entails a motion vector which describes the motion between the current block of a current dependent view and a block corresponding to this current block in a previously coded and then decoded reference view, the current dependent view and the base view each being representative of one and the same scene at the same instant.
More precisely, the MERGE mode allows the current block to directly inherit motion parameters, composed of an index of the reference image and of a temporal motion vector or else of a disparity motion vector for a neighbor block of the current block chosen from a list of candidates. The motion vector of the current block thus chosen by inheritance from the list of candidate vectors does not then require to be coded insofar as the same principle can be applied during the decoding. In such a case, only the index of the candidate which has been chosen and optionally prediction residuals are transmitted in the stream of coded data.
Being as it is 2D or 3D decoding, it is done image by image, and for each image, CTU block by CTU block. For each CU block of a CTU block, the corresponding elements of the stream are read. The inverse quantization and the inverse transformation of the coefficients of the CU blocks are performed. Next, the prediction of each CU block is calculated and each CU block is reconstructed by adding the predictor to the decoded prediction residual.
Intra or inter coding by competition, such as implemented in the HEVC standard, thus relies on putting various coding parameters, such as those aforementioned, into competition with the aim of selecting the best mode of coding, that is to say that which will optimize the coding of the block considered according to a predetermined performance criterion, for example the bitrate/distortion cost well known to the person skilled in the art.
Moreover, in the domain of video compression, video indexation or the detection of objects for example, optical flow is a motion estimation technique which produces a dense field of motion vectors, associated with a current image.
In video compression applications, optical flow makes it possible to produce a dense field of motion vectors, associated with a current image to be coded and, more particularly, with each current block to be coded, when the current image is partitioned into blocks. More precisely, for each pixel of the current block to be coded, a motion vector is calculated which points at an already coded and then decoded reference image. The field of these motion vectors thus calculated is termed dense since the pixels of the reference image that are pointed at by these motion vectors are not necessarily contiguous. Such a technique differs from the conventional techniques of “block matching” which are customarily present in most video coding norms, and which, for a current block to be coded, calculate a single motion vector describing the motion between the current block to be coded and an already coded and then decoded reference block. Said reference block is chosen as minimizing a coding performance criterion, such as for example the distortion criterion well known to the person skilled in the art.
The optical flow technique makes it possible to obtain a more precise Inter prediction of the blocks of an image. However, each motion vector belonging to the dense field of motion vectors is necessarily transmitted to the decoder, thereby substantially increasing the signaling cost and, therefore, compensates all the potential gains that one might hope to obtain with this technique.